Fe and O isotopes in coesite-bearing jadeite quartzite from the Western Alps record multistage fluid-rock interactions in a continental subduction zone

•Coesite firstly found in the granitic gneiss of the Dora Maira Massif provides direct evidence for the UHP metamorphism of country rocks.•Jadeite quartzite has a similar protolith of Permain granite to its host whiteschist.•The distinct Fe-O isotopes of jadeite quartzite are the result of complex fluid-rock interaction in the continental subduction zone.•Fe-O isotopes are powerful tracers for the fluid source and composition in the subduction zone.